The present invention relates to a method of manufacturing golf balls having a rubber core of one or more layer and a cover of one or more layer which encloses the core. More specifically, the invention relates to a method of manufacturing golf balls which is able to increase the sphericity of the rubber core and can use the core-forming rubber material without waste.
Generally, to mold a golf ball core, unvulcanized rubber is formed into a slug of a fixed weight by way of an extrusion step. The slug is then inserted into a vulcanizing mold, where it is subjected to vulcanization under pressure, thereby giving a rubber material in the form of a sphere. When a rubber sphere of two or more layers is produced by pressure vulcanization, it is common to pressure-vulcanize the inner rubber layer and subsequently carry out a step in which the inner rubber layer is enclosed by unvulcanized rubber in the form of hemispheres, following which the latter is subjected to vulcanization under applied pressure so as to create an outer rubber layer (envelope layer).
The following method is generally used to then render the above rubber sphere into a golf ball. That is, the core of one or more layer is set within a mold having dimple-forming features. A cover material is subsequently injection-molded over the core while the core is held by support pins or the like. After cooling, the molded body is removed from the mold, then stamped and clear-coated.
However, when compression-molding and vulcanizing the inner rubber layer (inner core layer) by the above method, it is necessary to insert and vulcanize under pressure an amount of rubber material which is greater than the mold volume, causing flash to form at the mold parting plane, as a result of which a pressurized condition is maintained. A deflashing and trimming step is required to remove the flash, but such a step inevitably results in a decline in sphericity and wasteful use of the material in those areas that become flash. Moreover, to provide the core with a layer construction, as noted above, unvulcanized rubber shaped into hemispheres must be furnished beforehand, thus increasing the number of operations. Furthermore, given the need to separately fashion such materials for the inner core layer and the outer core layer, dust and debris tend to adhere to the surfaces of the inner core layer and the outer core layer, resulting in a bond of inadequate strength between the two core layers.
Similarly, the complete formation of two or more cover layers (where “cover layer” refers generally to, for example, an envelope layer, intermediate layer and outermost layer) requires that, in the course of such formation, hemispherical sections be created for each layer, thus increasing the number of operations. Moreover, because the hemispherical sections are separately fabricated for each layer, the adherence of dust and debris weakens the strength of bonded areas therebetween.
Known methods of manufacturing golf balls include those disclosed in JP-A 8-322966 and JP-A 51-118535, although the core material is not injection-molded in these prior-art methods.